Like other projects, we also have some guidelines to keep to the

code. For Git in general, a few rough rules are:

- Most importantly, we never say "It's in POSIX; we'll happily

ignore your needs should your system not conform to it."

We live in the real world.

- However, we often say "Let's stay away from that construct,

it's not even in POSIX".

- In spite of the above two rules, we sometimes say "Although

this is not in POSIX, it (is so convenient | makes the code

much more readable | has other good characteristics) and

practically all the platforms we care about support it, so

let's use it".

Again, we live in the real world, and it is sometimes a

judgement call, the decision based more on real world

constraints people face than what the paper standard says.

- Fixing style violations while working on a real change as a

preparatory clean-up step is good, but otherwise avoid useless code

churn for the sake of conforming to the style.

"Once it _is_ in the tree, it's not really worth the patch noise to

go and fix it up."

Cf. http://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html

Make your code readable and sensible, and don't try to be clever.

As for more concrete guidelines, just imitate the existing code

(this is a good guideline, no matter which project you are

contributing to). It is always preferable to match the _local_

convention. New code added to Git suite is expected to match

the overall style of existing code. Modifications to existing

code is expected to match the style the surrounding code already

uses (even if it doesn't match the overall style of existing code).

But if you must have a list of rules, here they are.

For shell scripts specifically (not exhaustive):

- We use tabs for indentation.

- Case arms are indented at the same depth as case and esac lines,

like this:

case "$variable" in

pattern1)

do this

;;

pattern2)

do that

;;

esac

- Redirection operators should be written with space before, but no

space after them. In other words, write 'echo test >"$file"'

instead of 'echo test> $file' or 'echo test > $file'. Note that

even though it is not required by POSIX to double-quote the

redirection target in a variable (as shown above), our code does so

because some versions of bash issue a warning without the quotes.

(incorrect)

cat hello > world < universe

echo hello >$world

(correct)

cat hello >world <universe

echo hello >"$world"

- We prefer $( ... ) for command substitution; unlike ``, it

properly nests. It should have been the way Bourne spelled

it from day one, but unfortunately isn't.

- If you want to find out if a command is available on the user's

$PATH, you should use 'type <command>', instead of 'which <command>'.

The output of 'which' is not machine parseable and its exit code

is not reliable across platforms.

- We use POSIX compliant parameter substitutions and avoid bashisms;

namely:

- We use ${parameter-word} and its [-=?+] siblings, and their

colon'ed "unset or null" form.

- We use ${parameter#word} and its [#%] siblings, and their

doubled "longest matching" form.

- No "Substring Expansion" ${parameter:offset:length}.

- No shell arrays.

- No strlen ${#parameter}.

- No pattern replacement ${parameter/pattern/string}.

- We use Arithmetic Expansion $(( ... )).

- Inside Arithmetic Expansion, spell shell variables with $ in front

of them, as some shells do not grok $((x)) while accepting $(($x))

just fine (e.g. dash older than 0.5.4).

- We do not use Process Substitution <(list) or >(list).

- Do not write control structures on a single line with semicolon.

"then" should be on the next line for if statements, and "do"

should be on the next line for "while" and "for".

(incorrect)

if test -f hello; then

do this

fi

(correct)

if test -f hello

then

do this

fi

- If a command sequence joined with && or || or | spans multiple

lines, put each command on a separate line and put && and || and |

operators at the end of each line, rather than the start. This

means you don't need to use \ to join lines, since the above

operators imply the sequence isn't finished.

(incorrect)

grep blob verify_pack_result \

| awk -f print_1.awk \

| sort >actual &&

...

(correct)

grep blob verify_pack_result |

awk -f print_1.awk |

sort >actual &&

...

- We prefer "test" over "[ ... ]".

- We do not write the noiseword "function" in front of shell

functions.

- We prefer a space between the function name and the parentheses,

and no space inside the parentheses. The opening "{" should also

be on the same line.

(incorrect)

my_function(){

...

(correct)

my_function () {

...

- As to use of grep, stick to a subset of BRE (namely, no \{m,n\},

[::], [==], or [..]) for portability.

- We do not use \{m,n\};

- We do not use -E;

- We do not use ? or + (which are \{0,1\} and \{1,\}

respectively in BRE) but that goes without saying as these

are ERE elements not BRE (note that \? and \+ are not even part

of BRE -- making them accessible from BRE is a GNU extension).

- Use Git's gettext wrappers in git-sh-i18n to make the user

interface translatable. See "Marking strings for translation" in

po/README.

- We do not write our "test" command with "-a" and "-o" and use "&&"

or "||" to concatenate multiple "test" commands instead, because

the use of "-a/-o" is often error-prone. E.g.

test -n "$x" -a "$a" = "$b"

is buggy and breaks when $x is "=", but

test -n "$x" && test "$a" = "$b"

does not have such a problem.

For C programs:

- We use tabs to indent, and interpret tabs as taking up to

8 spaces.

- We try to keep to at most 80 characters per line.

- As a Git developer we assume you have a reasonably modern compiler

and we recommend you to enable the DEVELOPER makefile knob to

ensure your patch is clear of all compiler warnings we care about,

by e.g. "echo DEVELOPER=1 >>config.mak".

- We try to support a wide range of C compilers to compile Git with,

including old ones. You should not use features from newer C

standard, even if your compiler groks them.

There are a few exceptions to this guideline:

. since early 2012 with e1327023ea, we have been using an enum

definition whose last element is followed by a comma. This, like

an array initializer that ends with a trailing comma, can be used

to reduce the patch noise when adding a new identifer at the end.

. since mid 2017 with cbc0f81d, we have been using designated

initializers for struct (e.g. "struct t v = { .val = 'a' };").

. since mid 2017 with 512f41cf, we have been using designated

initializers for array (e.g. "int array[10] = { [5] = 2 }").

These used to be forbidden, but we have not heard any breakage

report, and they are assumed to be safe.

- Variables have to be declared at the beginning of the block, before

the first statement (i.e. -Wdeclaration-after-statement).

- Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"

is still not allowed in this codebase.

- NULL pointers shall be written as NULL, not as 0.

- When declaring pointers, the star sides with the variable

name, i.e. "char *string", not "char* string" or

"char * string". This makes it easier to understand code

like "char *string, c;".

- Use whitespace around operators and keywords, but not inside

parentheses and not around functions. So:

while (condition)

func(bar + 1);

and not:

while( condition )

func (bar+1);

- We avoid using braces unnecessarily. I.e.

if (bla) {

x = 1;

}

is frowned upon. But there are a few exceptions:

- When the statement extends over a few lines (e.g., a while loop

with an embedded conditional, or a comment). E.g.:

while (foo) {

if (x)

one();

else

two();

}

if (foo) {

/*

* This one requires some explanation,

* so we're better off with braces to make

* it obvious that the indentation is correct.

*/

doit();

}

- When there are multiple arms to a conditional and some of them

require braces, enclose even a single line block in braces for

consistency. E.g.:

if (foo) {

doit();

} else {

one();

two();

three();

}

- We try to avoid assignments in the condition of an "if" statement.

- Try to make your code understandable. You may put comments

in, but comments invariably tend to stale out when the code

they were describing changes. Often splitting a function

into two makes the intention of the code much clearer.

- Multi-line comments include their delimiters on separate lines from

the text. E.g.

/*

* A very long

* multi-line comment.

*/

Note however that a comment that explains a translatable string to

translators uses a convention of starting with a magic token

"TRANSLATORS: ", e.g.

/*

* TRANSLATORS: here is a comment that explains the string to

* be translated, that follows immediately after it.

*/

_("Here is a translatable string explained by the above.");

- Double negation is often harder to understand than no negation

at all.

- There are two schools of thought when it comes to comparison,

especially inside a loop. Some people prefer to have the less stable

value on the left hand side and the more stable value on the right hand

side, e.g. if you have a loop that counts variable i down to the

lower bound,

while (i > lower_bound) {

do something;

i--;

}

Other people prefer to have the textual order of values match the

actual order of values in their comparison, so that they can

mentally draw a number line from left to right and place these

values in order, i.e.

while (lower_bound < i) {

do something;

i--;

}

Both are valid, and we use both. However, the more "stable" the

stable side becomes, the more we tend to prefer the former

(comparison with a constant, "i > 0", is an extreme example).

Just do not mix styles in the same part of the code and mimic

existing styles in the neighbourhood.

- There are two schools of thought when it comes to splitting a long

logical line into multiple lines. Some people push the second and

subsequent lines far enough to the right with tabs and align them:

if (the_beginning_of_a_very_long_expression_that_has_to ||

span_more_than_a_single_line_of ||

the_source_text) {

...

while other people prefer to align the second and the subsequent

lines with the column immediately inside the opening parenthesis,

with tabs and spaces, following our "tabstop is always a multiple

of 8" convention:

if (the_beginning_of_a_very_long_expression_that_has_to ||

span_more_than_a_single_line_of ||

the_source_text) {

...

Both are valid, and we use both. Again, just do not mix styles in

the same part of the code and mimic existing styles in the

neighbourhood.

- When splitting a long logical line, some people change line before

a binary operator, so that the result looks like a parse tree when

you turn your head 90-degrees counterclockwise:

if (the_beginning_of_a_very_long_expression_that_has_to

|| span_more_than_a_single_line_of_the_source_text) {

while other people prefer to leave the operator at the end of the

line:

if (the_beginning_of_a_very_long_expression_that_has_to ||

span_more_than_a_single_line_of_the_source_text) {

Both are valid, but we tend to use the latter more, unless the

expression gets fairly complex, in which case the former tends to

be easier to read. Again, just do not mix styles in the same part

of the code and mimic existing styles in the neighbourhood.

- When splitting a long logical line, with everything else being

equal, it is preferable to split after the operator at higher

level in the parse tree. That is, this is more preferable:

if (a_very_long_variable * that_is_used_in +

a_very_long_expression) {

...

than

if (a_very_long_variable *

that_is_used_in + a_very_long_expression) {

...

- Some clever tricks, like using the !! operator with arithmetic

constructs, can be extremely confusing to others. Avoid them,

unless there is a compelling reason to use them.

- Use the API. No, really. We have a strbuf (variable length

string), several arrays with the ALLOC_GROW() macro, a

string_list for sorted string lists, a hash map (mapping struct

objects) named "struct decorate", amongst other things.

- When you come up with an API, document its functions and structures

in the header file that exposes the API to its callers. Use what is

in "strbuf.h" as a model for the appropriate tone and level of

detail.

- The first #include in C files, except in platform specific compat/

implementations, must be either "git-compat-util.h", "cache.h" or

"builtin.h". You do not have to include more than one of these.

- A C file must directly include the header files that declare the

functions and the types it uses, except for the functions and types

that are made available to it by including one of the header files

it must include by the previous rule.

- If you are planning a new command, consider writing it in shell

or perl first, so that changes in semantics can be easily

changed and discussed. Many Git commands started out like

that, and a few are still scripts.

- Avoid introducing a new dependency into Git. This means you

usually should stay away from scripting languages not already

used in the Git core command set (unless your command is clearly

separate from it, such as an importer to convert random-scm-X

repositories to Git).

- When we pass <string, length> pair to functions, we should try to

pass them in that order.

- Use Git's gettext wrappers to make the user interface

translatable. See "Marking strings for translation" in po/README.

- Variables and functions local to a given source file should be marked

with "static". Variables that are visible to other source files

must be declared with "extern" in header files. However, function

declarations should not use "extern", as that is already the default.

- You can launch gdb around your program using the shorthand GIT_DEBUGGER.

Run `GIT_DEBUGGER=1 ./bin-wrappers/git foo` to simply use gdb as is, or

run `GIT_DEBUGGER="<debugger> <debugger-args>" ./bin-wrappers/git foo` to

use your own debugger and arguments. Example: `GIT_DEBUGGER="ddd --gdb"

./bin-wrappers/git log` (See `wrap-for-bin.sh`.)

For Perl programs:

- Most of the C guidelines above apply.

- We try to support Perl 5.8 and later ("use Perl 5.008").

- use strict and use warnings are strongly preferred.

- Don't overuse statement modifiers unless using them makes the

result easier to follow.

... do something ...

do_this() unless (condition);

... do something else ...

is more readable than:

... do something ...

unless (condition) {

do_this();

}

... do something else ...

*only* when the condition is so rare that do_this() will be almost

always called.

- We try to avoid assignments inside "if ()" conditions.

- Learn and use Git.pm if you need that functionality.

- For Emacs, it's useful to put the following in

GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:

;; note the first part is useful for C editing, too

((nil . ((indent-tabs-mode . t)

(tab-width . 8)

(fill-column . 80)))

(cperl-mode . ((cperl-indent-level . 8)

(cperl-extra-newline-before-brace . nil)

(cperl-merge-trailing-else . t))))

For Python scripts:

- We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).

- As a minimum, we aim to be compatible with Python 2.6 and 2.7.

- Where required libraries do not restrict us to Python 2, we try to

also be compatible with Python 3.1 and later.

- When you must differentiate between Unicode literals and byte string

literals, it is OK to use the 'b' prefix. Even though the Python

documentation for version 2.6 does not mention this prefix, it has

been supported since version 2.6.0.

Error Messages

- Do not end error messages with a full stop.

- Do not capitalize ("unable to open %s", not "Unable to open %s")

- Say what the error is first ("cannot open %s", not "%s: cannot open")

Externally Visible Names

- For configuration variable names, follow the existing convention:

. The section name indicates the affected subsystem.

. The subsection name, if any, indicates which of an unbounded set

of things to set the value for.

. The variable name describes the effect of tweaking this knob.

The section and variable names that consist of multiple words are

formed by concatenating the words without punctuations (e.g. `-`),

and are broken using bumpyCaps in documentation as a hint to the

reader.

When choosing the variable namespace, do not use variable name for

specifying possibly unbounded set of things, most notably anything

an end user can freely come up with (e.g. branch names). Instead,

use subsection names or variable values, like the existing variable

branch.<name>.description does.

Writing Documentation:

Most (if not all) of the documentation pages are written in the

AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and

processed into HTML and manpages (e.g. git.html and git.1 in the

same directory).

The documentation liberally mixes US and UK English (en_US/UK)

norms for spelling and grammar, which is somewhat unfortunate.

In an ideal world, it would have been better if it consistently

used only one and not the other, and we would have picked en_US

(if you wish to correct the English of some of the existing

documentation, please see the documentation-related advice in the

Documentation/SubmittingPatches file).

Every user-visible change should be reflected in the documentation.

The same general rule as for code applies -- imitate the existing

conventions.

A few commented examples follow to provide reference when writing or

modifying command usage strings and synopsis sections in the manual

pages:

Placeholders are spelled in lowercase and enclosed in angle brackets:

<file>

--sort=<key>

--abbrev[=<n>]

If a placeholder has multiple words, they are separated by dashes:

<new-branch-name>

--template=<template-directory>

Possibility of multiple occurrences is indicated by three dots:

<file>...

(One or more of <file>.)

Optional parts are enclosed in square brackets:

[<extra>]

(Zero or one <extra>.)

--exec-path[=<path>]

(Option with an optional argument. Note that the "=" is inside the

brackets.)

[<patch>...]

(Zero or more of <patch>. Note that the dots are inside, not

outside the brackets.)

Multiple alternatives are indicated with vertical bars:

[-q | --quiet]

[--utf8 | --no-utf8]

Parentheses are used for grouping:

[(<rev> | <range>)...]

(Any number of either <rev> or <range>. Parens are needed to make

it clear that "..." pertains to both <rev> and <range>.)

[(-p <parent>)...]

(Any number of option -p, each with one <parent> argument.)

git remote set-head <name> (-a | -d | <branch>)

(One and only one of "-a", "-d" or "<branch>" _must_ (no square

brackets) be provided.)

And a somewhat more contrived example:

--diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]

Here "=" is outside the brackets, because "--diff-filter=" is a

valid usage. "*" has its own pair of brackets, because it can

(optionally) be specified only when one or more of the letters is

also provided.

A note on notation:

Use 'git' (all lowercase) when talking about commands i.e. something

the user would type into a shell and use 'Git' (uppercase first letter)

when talking about the version control system and its properties.

A few commented examples follow to provide reference when writing or

modifying paragraphs or option/command explanations that contain options

or commands:

Literal examples (e.g. use of command-line options, command names,

branch names, URLs, pathnames (files and directories), configuration and

environment variables) must be typeset in monospace (i.e. wrapped with

backticks):

`--pretty=oneline`

`git rev-list`

`remote.pushDefault`

`http://git.example.com`

`.git/config`

`GIT_DIR`

`HEAD`

An environment variable must be prefixed with "$" only when referring to its

value and not when referring to the variable itself, in this case there is

nothing to add except the backticks:

`GIT_DIR` is specified

`$GIT_DIR/hooks/pre-receive`

Word phrases enclosed in `backtick characters` are rendered literally

and will not be further expanded. The use of `backticks` to achieve the

previous rule means that literal examples should not use AsciiDoc

escapes.

Correct:

`--pretty=oneline`

Incorrect:

`\--pretty=oneline`

If some place in the documentation needs to typeset a command usage

example with inline substitutions, it is fine to use +monospaced and

inline substituted text+ instead of `monospaced literal text`, and with

the former, the part that should not get substituted must be

quoted/escaped.